Abstract: With importance index for modes of irregular continuous bridges as the object of study,the participation mass factor is firstly theoretically analyzed,and is considered that it has defects when estimating the importance of modes. And thus,the new indexes including a contribution displacement,a contribution displacement factor and a contribution displacement ratio are defined. And then,taking one irregular continuous bridge as an example,the participation mass factor and contribution displacement factor are investigated,considering that the contribution displacement ratio is more reasonable when estimating the importance of modes. Finally,the parameter analyses based on many continuous bridges validate that contribution displacement ratio has more advantages than participation mass factor as the importance index for modes. Results show that the participation mass factor can express the importance of modes for the structural total dynamic response; however,it can not express the importance of modes for the dynamic response of a local position. Because the contribution displacement ratio can mend the defects,it is more reasonable as the importance index for modes of irregular continuous bridges. When the sum of the former n modal contribution displacement ratios is beyond 80%,the calculated structure displacement based on the former n modes can satisfy the precision demand of irregular continuous bridges.